1. Field of the Invention
The present invention relates to equipment employable for detecting the concentration of oxygen contained in a gas, and more particularly to a limiting electric current type oxygen concentration detector which is provided with a temperature compensation means which applies temperature compensation to results of measurement for the concentration of oxygen contained in a gas.
The oxygen concentration detector in accordance with the present invention is employable for detecting the oxygen concentration contained in exhaust gases emitted by a boiler installed in thermal power stations, internal combustion engines mounted on automobiles.
2. Statement of the Prior Art
Needless to emphasize, various types of combustion equipment such as boilers installed in thermal power-stations, internal combustion engines and the like are presently available and they are means essential for modern social life in various aspects.
However, such equipment is inevitably accompanied by a possibility of emission of a considerable quantity of injurious gases, depending on the condition on which such combustion equipment operates or the condition on which the combustion takes place. Further, a strong requirement has come out for development of combustion equipment which requires less fuel consumption.
It is believed that combustion in an atmosphere containing a lesser quantity of a fuel (Hereinafter referred to as an air-fuel mixture or lean mixture) is possibly effective to simultaneously satisfy both the requirements to decrease the quantity of injurious gases contained in exhaust gases and to decrease the quantity of fuel used for generation of a unit quantity of energy. For example, it is publicly known that a lean mixture is preferably employed for a Diesel engine. Therefore, it is hopefully assumed that a lean mixture could be employed also for a gasoline engine to satisfy the foregoing objects.
However, a mixture containing a fuel and air in an undesirable ratio readily causes such engines to exhaust a considerable quantity of soot and/or causes misfiring, despite the fact that such engines inherently prefer lean mixture, resulting various problems including air pollution due to emission of soot or unburned fuel and less satisfactory combustion efficiency. In other words, such a misuse of a lean mixture is not only ineffective to satisfy the foregoing objects but also is involved with the possibility of causing various reverse effects. Therefore, adjustment of the ratio of fuel and air is an extremely important parameter to enable combustion equipment to operate under satisfactory conditions under which it is allowed to exhibit the expected performance. It is quite true for any type of control system that the accurate and quick detection of an object to be controlled (In this case, this object is the ratio of a mixture, and air in a lean fuel more specifically, the oxygen concentration in the exhaust gas.) is essential for performance of the control system. Unfortunately, however, none of the sensors which are at present available in the prior art is satisfactory for such purposes. For example, the magnetic oxygen concentration detector is unsatisfactory for the purpose to be employed under a condition wherein the detector is mounted on an automobile, because of its rather slow response speed. The density type sensor or thermal conductivity type sensor is also unsatisfactory for the purpose to control combustion of an internal combustion engine, because the accuracy thereof is inclined to be adversely influenced by a marginal quantity of hydrogen (H.sub.2) contained in a gas.
One piece of equipment for detecting oxygen concentration in accordance with the prior art is represented by a sensor employable for detecting limiting electric current for the purpose of analyzing oxygen concentration in a gas (Hereinafter, referred to as "a limiting electric current type oxygen concentration sensor"), the sensor being invented by the inventors of the present invention and was laid open to the public inspection under the laying-open of Application No. Toku-Kai-Sho No. 52-72286 in Japan and which disclosed the conceptual construction of the limiting electric current type oxygen concentration detector, and by another limiting electric current type oxygen concentration sensor which was invented by the inventors of the present invention and was filed under Application No. Toku-Gan-Sho No. 55-123677 in Japan which disclosed an improvement applied to the foregoing conceptual construction or an improved construction of a limiting electric current type oxygen concentration sensor of which the cathode is covered by a porous material layer.
Either of these limiting current type oxygen concentration sensors is free from various drawbacks which inevitably are involved with the oxygen concentration detectors available in the prior art. From this view point, either of these limiting current type oxygen concentration sensors is recognized as oxygen concentration sensor having an excellent features. From other viewpoints, particularly in the realistic aspects, however, either of the foregoing limiting current type oxygen concentration sensors is involved with possibilities of further improvements. It is quite often that internal combustion engines mounted on vehicles vary the temperature of their exhaust gases depending on the corresponding operating conditions. Therefore, a limiting current type oxygen concentration sensor is required to have a stable performance in a relatively wide range of temperature, when it is employed as a sensor for the exhaust gas of engines whose operation rates frequently vary e.g. engines mounted on vehicles. However, the limiting current type oxygen concentration sensors available in the prior art are accompanied by two drawbacks including (1) that the internal resistance of the sensor varies over a wide range depending on the temperature thereof, and (2) that the limiting electric current vs. oxygen concentration relations vary according to the temperature thereof, these drawbacks, in combination, resulting in an error of measurement of the oxygen concentration.